Underlayment requirements for 18x18 Porcelain tile

I'm installing 180 Sq feet of 3/8"porcelain tile in a kitchen. Ideally I would like the transition of the tile floor from the kitchen entrance to the hardwood flooring in the hallway as flush as possible. I removed all previous flooring down to the original 10" x 3/4" lap boards and left those due to electrical conduit running at that level. The home is about 60 years old. I have 1-1/2" inches to make up and have already installed 3/4" ply to get above the conduit. I now only have 3/4" left. I purchased 1/2" durock, but am considering returning that and going with 5/16" durock instead. I need to allow roughly 3/8" for thinset using a 1/2"x1/2" trowel. My question, after all that. Is the structural integrity of 5/16" durock able to support an 18x18 tile floor which will ultimately give me a flush transition the the Hardwwood flooring. OR. Do I stay with the 1/2" durock and install a transition threshold?
Thanks in Advance
Divo

The thickness of the tile backer adds no strength, so any size will work, 1/4" whatever, I would install Ditra instead of the CBU and use a 3/8" trowel to set the tile after the Ditra is down.

First of all thanks for the reply secondly I'm Not familiar with Ditra, why do you recommend this thinset instead of the versabond?
Divo

Ditra is an underlayment for tile, not a thinset.

http://www.schluter.com/6_1_ditra.aspx

I would use Ditra instead of cement board too, it's better and it only adds 1/8" installed so you can add thicker or more ply to gain strength. Downside? May be harder to find and it costs more. Let us know which way you go cuz the recommended thinset will be different.

A 1/4x3/8x1/4 trowel may be OK, but I would be using a 1/4x1/2x1/4 or 1/2x1/2 for those large tiles. The thin set will add 1/8" at the most, probably more like 3/32". So recalculate those figures.

Jaz

Thanks Rd and Jaz, I watched the Schluter installation video from their site, thanks Rd!. I will try to find a distributor in my area who carries it. I'll let you know what I decide.
Divo

Hi Guys!
Could not find a distributor in my area that carried Ditra, so I used 5/16 Durock worked out find used Versabond thinset as well. Thanks for the suggestions and advice. While working at this home a neighbor approached me to do 500 sq ft of Travertine in his basement over a heated floor. I'm excited for that opportunity.

Could not find a distributor in my area that carried Ditra,

SAY WHAT?

Home Depot sells it as well as every tile store in America having access to it.